Eye center retraining system and method

ABSTRACT

A system and method is provided for bit eye center retraining. In general, the system samples an incoming data stream to determine where transitions in the data stream occur, selectively compares the location of the transitions to the expected locations to produce difference values, and combines pairs of difference values to determine when the sample point of the data stream needs to be adjusted.

FIELD OF THE INVENTION

This invention generally relates to communication, and more specificallyrelates to eye center determination in communication systems.

BACKGROUND OF THE INVENTION

Communication systems are becoming increasingly important in all aspectsof business and personal life. Modem communication systems transmit dataat very high rates of speed. The data is typically transmitted as aseries of bits represented by high or low voltage levels in a signal,commonly called a bit stream. In these systems the receiver must be ableto accurately and reliably decode the bit stream to extract thetransmitted data.

One difficulty in extracting transmitted data from a bit stream isdetermining where each bit resides in the stream. The location of eachbit in the bit stream needs to be identified to determine where thereceiver should sample the data stream to accurately extract the data.Otherwise the receiver could miss data bits, sample the same data bittwice, misinterpret a data bit, or otherwise corrupt the received data.

This task is made more difficult and critical by the presence of noisein the communication system which can distort the shape of the datastream. Furthermore, the bit stream can commonly include consecutivebits that are either all high or all low, with no transitions betweenconsecutive bits. Even where transitions between bits exist it can bedifficult to quickly and accurately determine where the center of eachbit resides. The problem of locating bits in a bit stream is commonlyreferred to as training to find “eye center” of bits in a data stream.

This problem is particularly acute in burst mode communication devices,where individual clients are assigned specific time slices in which theybroadcast short bursts of data. In these systems the receiver must beable to accurately determine the location of bits for each burst ofdata. Typically there are only a few bits of preamble available in eachburst upon which this determination can be made, before actual data bitsthat must be sampled begin to arrive.

When the bit eye center has been located during the preamble there maystill be a need to adjust the bit eye center during the remainder of theburst. Specifically, during the burst, the eye center of the data streamcan drift from the original located position in the data stream. Thiscan be caused by a variety of factors. For example, small differences inclock speed between the transmitter and the receiver can cause the eyecenter of bits in the burst to drift over time. When this drift issevere, the sampling point can move unacceptably close to thetransitions, causing potential errors in the data sampling and recovery.To avoid these potential errors, it is desirable to adjust the locationof the sampling to compensate for any drift that is occurring. This isgenerally referred to as retraining to find a new bit eye center orsample point. In many systems it is required to retrain to avoidunacceptable data losses that would otherwise occur.

BRIEF DESCRIPTION OF DRAWINGS

The preferred exemplary embodiment of the present invention willhereinafter be described in conjunction with the appended drawings,where like designations denote like elements, and:

FIG. 1 is a schematic view of an eye center trainer in accordance withan embodiment of the invention;

FIGS. 2-5 are schematic views of data streams in accordance with anembodiment of the invention;

FIG. 6 is a schematic view of an offset procedure in accordance with anembodiment of the invention;

FIG. 7 is a schematic views of exemplary data sets in accordance with anembodiment of the invention;

FIG. 8 is a schematic view of a training system in accordance with anembodiment of the invention;

FIG. 9 is a schematic view of a retrain controller in accordance with anembodiment of the invention; and

FIGS. 10-13 are schematic views of a transition bit sets and a registerin accordance with an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a system and method for bit eye centerretraining. In general, the system samples an incoming data stream todetermine where transitions in the data stream occur, selectivelycompares the location of the transitions to their expected locations toproduce difference values, and combines pairs of difference values todetermine when the sample point of the data stream needs to be adjusted.

Turning now to FIG. 1, an eye center retrainer 100 is illustratedschematically. The eye center retrainer 100 includes a sampler 102, aretrain controller 104 and a data buffer 106. In general, the eye centerretrainer 100 receives a data stream and determines if the current biteye center used to sample the data stream is still valid, anddynamically adjusts the bit eye center to more accurately sample thedata stream. Specifically, the data stream is received by the sampler102. The sampler 102 samples the incoming data stream and passes thesamples to the retrain controller 104. The retrain controller 104 alsoreceives the current bit eye center being used to sample the datastream. The bit eye center corresponds to a recurring location in thebit stream that is in center of the bits, between any transitions, andis the location where samples of the data stream are taken to recoverdata from the data stream. The retrain controller 104 receives thesamples from the sampler 102 and determines where transitions in thedata stream occur, compares the location of the transitions to theexpected locations based on the current bit eye center to producedifference values. The retrain controller 104 combines pairs ofdifference values to determine when the sampling point of the datastream needs to be adjusted. The retrain controller 104 selectivelyadjusts which sample points are used by the data buffer to sample thedata stream. Thus, the eye center trainer 100 is able to retrain thesystem to more accurately sample from the center of the bit eyes in thedata stream.

The eye center retrainer 100 provides the ability to retrain to the eyecenter even in the case of noise in the system, whether the noise israndom or deterministic. One common type of deterministic noise isodd/even noise. Odd/even noise occurs when a component of a systemreacts differently to 0 to 1 transitions than it reacts to 1 to 0transitions. For example, when a 0 to 1 transition takes 1.0 ns while a1 to 0 transition takes 0.5 ns, 0 bits appear to be much wider than 1bits even though they are supposed to have the same duration. Odd/evennoise is common to many types of systems. The result of odd/even noisein the data stream is that some transitions appear early while theopposite transitions appear late. This results in some bits that areshortened while other bits are lengthened. Odd/even noise can be causedby a variety of factors, such as limitations in the amplifiers used todrive the data stream. Specifically, because of responsivenessconstraints in the amplifier, the upper and or lower portions of thedata stream are clipped in a way which effectively offsets the datastream waveform, shortening some bits and lengthening others. Thisdistortion of the data stream wave pattern makes accurate determinationof the eye center extremely difficult using traditional techniques. Aswill be described in greater detail below, the retrainer 100 uses pairsof transitions to adjust the sample point. Because the retrainer 100averages the locations of the transitions the effects of odd/even noiseare negated.

Specifically, the retrainer 100 uses pairs of transitions, and thus oneedge (rising or falling) will be earlier than expected by some amount,and the other edge (falling or rising) will be later than expected bythe same amount. When the transition locations for rising and fallingedges are paired, a constant amount is effectively subtracted from halfthe numbers and the same amount is added to the remaining half. Thus,the resulting average is not affected by the odd/even noise and theretrainer 100 is able to accurately determine the eye center of bits inthe data stream and adjust the sample point accordingly. Other types ofnoise of may move individual edges with no relationship to other edges.

Turning now to FIG. 2, an exemplary incoming data stream is illustratedin graph 200. The data stream is sampled by sampler 102 to locate thetransitions in the data stream. The location of those transitions isthen used to determine the eye center of bits in the data stream. Thelocations of initial transitions in a data burst are used for theinitial training, the initial determination of the eye center used torecover data from the data stream. After the initial training, laterdata bits are sampled for retraining. Specifically, the later data bitsare sampled to determine the locations of transitions in the datastream, and those determined locations are compared to the locationsthat were expected based on previous training. Based on the comparisons,the retraining system selectively adjusts the eye center, thus adjustingthe sample points that are used by the data buffer to sample the datastream.

The data stream samples are grouped in sets of samples. In the exampleillustrated in FIG. 2, each sample set corresponds to the bit time ofthe data stream. It should be noted that this is just one example, andin some cases, typically depending on the frequency of the data stream,each sample set may include more or less of the data stream. In theillustrated example, each sample set includes 16 separate samples.Again, this is just one example, and in some cases more or less sampleswill be used.

One specific embodiment of a training system used to determine theinitial location of the bit eye centers in the data stream will now bediscussed with reference to FIGS. 3-7. The training system is an exampleof the type of system that can be used to determine the initial bit eyecenter to the retrain controller 104.

In general, the training system receives a data stream, samples theincoming data stream, and uses the samples to locate transitions. Tapnumbers corresponding to the located transitions are offset based onstate criteria and the number of transitions in a bit time of the datastream. The criteria and offsets are selected to facilitate accumulationof the offset tap numbers such that the tap numbers can be easilyaveraged to locate eye center of the bits. The initial training isperformed using a preamble of the data stream. Specifically, thepreamble provides a set of bits used to determine the center of the biteye such that the later data bits in the data stream can be reliablydecoded.

Turning now to FIGS. 3-5, several exemplary incoming data stream areillustrated. The data stream is sampled by sampler to locate thetransitions in the data stream. The data stream samples are grouped insets of samples. As will be described in greater detail below, the setsof samples are grouped together to facilitate analysis by the traincontroller 104. In the example illustrated in FIG. 3, each sample setcorresponds to one bit time of the data stream. It should be noted thatthis is just one example, and in some cases, typically depending on thefrequency of the data stream, each sample set may include more or lessof the data stream.

The samples provided by the sampler are identified by tap number, wherea tap comprises the channel through which the sample of the bit streamis conveyed from the sampler. Each tap provides samples at differenttimes in the data stream. The tap number identifies which tap, and thus,where in time a particular sample was taken.

Typically, the sampler uses a clock to determine when to sample the bitstream. The rate of the sampler clock and the number of samples takenduring each sampler clock period determine the number of samples takenof the data stream. In this application, the sampler will be describedas producing N samples per sampler clock period. It should be noted thatthe sampler clock period could be the same as the data stream period, orit could be different. It should also be understood that the samplerclock period may not correspond to an actual clock period, and couldinstead comprise multiple actual clock periods or a portion of theactual clock period. Furthermore, as will be described in greater detailbelow, all or a portion of the N samples per sampler clock period can beused to locate the eye center.

In this application T is the “width” of a data bit on the data stream asmeasured by the number of taps which sample each bit in the data stream.T is thus determined by the rate of sampling and the rate of theincoming data stream. Again, the rate of sampling would typically bedependent upon a clock rate of the sampler used to take the samples. Forexample, a typical sampler may be able to take 32 samples per samplerclock cycle. Thus, if a sampler can take 32 samples per sampler clockperiod and there are four sampler clock periods per data bit, T would beequal to 128. Likewise, if there is only one sampler clock period perdata bit, T would be equal to 32. If there were two data bits for eachsampler clock period then T would equal 16. Finally, if there were fourdata bits for each sampler clock period then T would equal 8.

Depending on the time between taps and the number of taps per bit T,some portion of the N taps can be used during each sampler clock period.In one embodiment, in order to simplify device logic the same number ofsamples is used per bit for several different incoming data streamrates.

As one specific example, assume a system with a sampler clock rate of622 MHz and 32 taps per sampler clock period. In this system the samplescan be selected to use 16 samples for 155 MHz, 622 MHz, and 1.2 GHz,while 8 samples are used for 2.4 GHz. Specifically, for an incoming datarate of 155 MHz only 4 of 32 taps are used per sampler clock cycle.Because there are four 622 MHz sampler clock cycles for each 155 MHzdata cycle, a total of 16 samples out of 128 are thus used to locate theeye center. Likewise, for an incoming data rate of 622 MHz only 16 of 32taps are used. Because there is one 622 MHz sampler clock cycle for each622 MHz data cycle, a total of 16 samples out of 32 are thus used tolocate the eye center. Likewise, for an incoming data rate of 1.2 GHzall taps are used. Because there are two 1.2 GHz data cycles for each622 MHz sampler clock cycle a total of 16 samples are thus used tolocate the eye center. Finally, for an incoming data rate of 1.2 GHz alltaps are used. Because there are four 2.4 GHz data cycles for each 622MHz sampler clock cycle a total of only 8 samples are used to locate theeye center. Thus, this method uses 16 samples per data bit for the threeslowest data rates, thus simplifying device complexity while providingacceptable accuracy. For the fastest rate of 2.4 GHz the method uses 8samples per data, an acceptable compromise for accuracy and complexity.

In the examples illustrated in FIGS. 3-5, three exemplary incoming datastreams are illustrated. These three examples show how data rates ofdifferent speed can be sampled by one sampler. In the three examples thesampler uses the same sampler clock period for all three data rates.Additionally, in all three examples the samples are grouped into sets.Specifically, each sample set corresponds to selected samples that spanone bit time of the data stream, with each of the sample sets having thesame number of samples. As described above, using sample sets of thesame size can reduce device complexity by simplifying the logic neededto handle the different data rates.

Referring now specifically to FIG. 3, an exemplary incoming data streamis illustrated in graph 210. In the example illustrated in FIG. 3, 32samples are taken during each sampler clock period, which is equal to abit period in this example. Thus N (the number of samples per samplerclock period) and T (the width of the data bit in samples) are both 32.However, in this example only a portion of the samples (16) are includedin the sample set that corresponds to one bit time, and will be used fortraining. The 16 samples would each be identified by the tap number,where the tap numbers correspond to the taken 32 samples. Thus, the 16samples used would correspond to tap numbers 0, 2, 4, 6, 8, etc.

Turning now to FIG. 4, another exemplary incoming data stream isillustrated in graph 220. In this example the sample rate is the same asin FIG. 3, but the data stream rate is twice as fast. In the exampleillustrated in FIG. 4, 32 samples are taken during each sampler clockperiod, which is equal to two bit periods. Thus N (the number of samplesper sampler clock period) is 32 while T (the width of the data bit insamples) is 16. Again, like the previous example, there are 16 samplesselected for each sample set that corresponds to one bit time. The 16samples would each be identified by the tap number, where the tapnumbers correspond to the taken 32 samples in each sampler clock period.Thus, the 16 samples used would correspond to tap numbers 0, 1, 2, 3, 4,etc.

Turning now to FIG. 5, another exemplary incoming data stream isillustrated in graph 230. In this example the sample rate is the same asin FIG. 3, but the data stream rate is a quarter as fast. In the exampleillustrated in FIG. 4, 32 samples are taken during each sampler clockperiod, which is equal to one-quarter of a bit period. Thus, N (thenumber of samples per sampler clock period) is 32 and T (the width ofthe data bit in samples) is 128. Again, there are 16 samples selected ineach sample set. The 16 samples would each be identified by the tapnumber, where the tap numbers correspond to the taken 32 samples in eachsampler clock period. Thus, the 16 samples used would correspond to tapnumbers 0, 8, 16, 24 . . . 120, etc.

FIGS. 3-5 thus illustrate how incoming data streams of different ratescan be sampled by one sampler, and how sample sets of the same size canbe grouped together and referenced by tap number.

As described above, the initial eye center training receives theincoming data stream into a sampler that samples the data stream. Thosesamples are then used to locate the transitions in the data stream.Typically, the transitions would be located by determining which samplesare adjacent to the transitions. When one sample shows a high value, andthe next sample shows a low value, then a transition from high to lowhas occurred between the two samples. Likewise, when one sample shows alow value, and the next sample shows a high value, then a transition lowto high has occurred between the two samples. Thus, the system canlocate which samples correspond to the location of the transitions, andcan identify those samples by their tap number.

The trainer then identifies the samples proximate to the transitions,and offsets the tap numbers for those samples based on state criteriaand the number of transitions in a bit time, and loads the offset tapnumbers into an accumulator. As one example, the trainer offsets the tapnumbers which most closely follows the transition an amount determinedby the current state and whether the transition is the first or secondtransition in the bit time.

The criteria and offsets used by the trainer are selected to facilitateaccumulation of the offset tap numbers in the accumulator in such a waythat the eye center of the bits is easily located. The offsets areselected to facilitate averaging of tap values without the problems thatwould otherwise occur for transitions that are early or late. Ingeneral, the system offsets tap values corresponding to transitions thatare “on time” (i.e., in the sample set of the bit time in which theywere expected) by a value T, where T is the number of samples in a bittime. Tap values corresponding to transitions that are “early” (i.e., inthe bit time before they were expected) are offset by a value of 0. Tapvalues corresponding to transitions that are “late” (i.e., in the bittime after they were expected) are offset by a value of 2T.

Stated another way, the tap values are offset an amount O, whereO=(S−X+1)T, where S is the bit time “number” for the tap value, X is thetransition number, and T is the number of samples in a bit time. Thus,the first transition in the first set of samples would be offset(1−i+1)T or T. If the second transition arrives early, also in the firstbit time, it would be offset (1−2+1)T or 0. If instead, the secondtransition arrives in the second bit time it would be offset (2−2+1)T orT. Finally, if instead the second transition does not show up until thethird bit time it would be offset (3−2+1)T or 2T. This process starts atthe first transition in the training data and is continued untilsufficient transitions are located, offset and accumulated to determinethe location of the bit eye center.

Turning now to FIG. 6, a state diagram 300 illustrates a state machineimplementation of the above procedure that selectively offsets selectedtap numbers. The offset of the tap numbers facilitates easy location ofthe eye center from the accumulator. The procedure 300 includes threestates, a normal state 302, a double edge state 304, and a zero edgestate 306. Each state determines how the tap numbers are offset based onthe number of transitions in its set of samples that corresponds to onebit time. In this specific example, the tap numbers can be offset byamounts of 0, T, or 2T, where T is the number of samples in the sampleset. For example the tap number corresponding to the first edge (i.e.,transition) in a set of samples received in the normal state 302 isoffset to between T and 2T−1 by adding T to the tap number.

The procedure 300 moves between states based on the number oftransitions (referred to as edges in FIG. 3) found in the set of samplesthat corresponds to a bit time. Thus, when the procedure has seen a setof samples that equals the width of a bit in the data stream, theprocedure transitions to the next state. For example, if while in thenormal state 302 a set of samples has one transition the procedure 300stays in the normal state 302. However, if while in the normal state 302a set of samples has two transitions, the procedure moves to double edgestate 304. If while in the double edge state 304 a set of samples hasone transition, the procedure 300 stays in the double edge state 304.Conversely, if while in the double edge state 304 a set of samples hasno transition, the procedure 300 moves to the normal state 302. Itshould be noted that while in the double edge state 304 a secondtransition will not be received in a set of transitions because thatwould require more transitions than are possible to receive within thebit times.

If while in the normal state 302 a set of samples has no transitions,the procedure 300 moves to the zero edge state 306. If while in the zeroedge state 306 a set of samples has two transitions, the procedure 300returns to the normal state 302. Finally, if while in the zero edgestate 306 a set of samples has one transition, the procedure 300 staysin the zero edge state 306.

In each case, the tap numbers corresponding to received transitions areoffset according to the rules of that state, and the offset tap numbersare added to the contents of accumulator 106. Those rules are dependentupon the number of the transitions in the set. Thus, while in the normalstate, the tap number corresponding to the first transition of a set ofsamples is offset to the range T:2T−1 by adding T. If a secondtransition is located in that set of samples, its corresponding tapnumber is given an offset of 0 to remain in the range of 0:T−1 (and theprocedure would move to double edge state 304).

As another example, while in the zero edge state 306, the tap numbercorresponding to the first transition of a set of samples is offset tobe in the range of 2T:3T−1. If a second transition is then located inthat set of samples, its corresponding tap number is offset to the rangeof T:2T−1 (and the procedure 300 would move to the normal state 302).

During operation, the procedure 300 starts in the normal state 302 whenthe first transition in the preamble training bits is located by thetrainer. Specifically, the trainer would typically be configured tocontinuously sample and monitor the signal for incoming data. When a newtransmission of data arrives, the only bits to be passed to the eyecenter trainer are the preamble training bits. When the first transitionin the preamble training bits is sampled, the trainer locates thetransition and offset according to the rule for edge 1 in the normalstate. Thus, the tap number for the first transition located by thecontroller is offset to the range T:2T−1. If a second transition isfound in the set of samples, that corresponding tap number is offsetaccording to the edge 2 rule in the normal state, and the proceduremoves to double edge state 304. This process continues until a selectednumber of transitions are located and their tap numbers are offset andadded into the accumulator.

As described above, the criteria and offsets used by procedure 300 aredesigned to facilitate accumulation of the offset tap numbers in theaccumulator in such a way that the eye center of the bits is easilylocated. Specifically, in this embodiment the eye center of the bits canbe easily located using the higher order bits of the accumulated values.

In the general sense the accumulator determines the location of the eyecenter by averaging the location of the transitions, as offset accordingto the procedure described above. The result is a value that correspondsto the average location of the transition in the bit stream. From this,the eye center of the bit for the data stream can be easily calculatedby selecting the tap number that is one-half a bit time away from theaverage transition location. Turning now to FIG. 7, three exemplarytraining data sets 402, 422 and 442 are illustrated graphically, alongwith the examples of accumulating their offset values. In theseexamples, each training data set includes four sample sets, with eachsample set corresponding to one bit time and including 16 samples, eachsample being identified by a tap number of between 0 and 15, and T has avalue of 16.

The samples immediately following a detected transition are indicatedwith shaded boxes. For example, in data set 402, transitions werelocated in the sample at tap number 7 of sample set 1, tap number 8 ofsample set 2, tap number 6 of sample set 3 and tap number 7 of sampleset 4. In this relatively simple example there is one transition inevery sample set. In procedure 300 the method starts in the normal state302, and thus the first transition in set 1 would be offset to the rangeT:2T−1. Because there is only one transition in each of the remainingsample sets the procedure 300 would stay in the normal state 302,meaning that each transition would be offset by the same value of 16.

The tables 410 and 411 illustrate the result of this accumulation.Specifically, the table 410 includes a column 404 of the four measuredtap numbers, while column 406 shows the four corresponding offsets. Thetable 410 shows the values in binary, as they would be stored andprocessed by the accumulator. The first number in column 404, 0111, is atap number of 7. Prefixing bits to 01 to this number results in anoffset tap number of 010111, effectively offsetting the tap number by16. Table 411 shows the running values in the accumulator when theappropriate offset is prefixed to each of the tap numbers and added intothe accumulator. As illustrated in entry 408, the result of adding thevalues into the accumulator is 1011100. Because of the way the offsetvalues were selected, the average of all four tap numbers can bedetermined using a relatively simple bit shift. Specifically, theaverage of all four tap numbers can be obtained by dropping the lowesttwo significant bits and using the next four significant bits 410.Dropping the lowest two significant bits corresponds to dividing thevalue in the accumulator by four, and thus results in the averagetransition tap number. Thus, the average of the four tap numbers is0111, which corresponds to a value of 7. This average of the four tapnumbers corresponds to the average location of the transition in the bitstream. To locate the eye center of data in the bit stream, this valueis translated one half of a bit width, from the location of thetransition to the eye center. This shift can be done by adding orsubtracting the appropriate value to the accumulator output.Alternatively, this shift can be compensated for automatically byseeding the accumulator with an initial value, as will be described ingreater detail below. Thus, by simply offsetting and accumulating fourmeasured tap numbers the location of the eye center can be quicklyobtained.

While example data set 402 illustrates a simple example of how procedure300 works, it does not fully illustrate how procedure 300 deals withmultiple transitions (or no transitions) in a sample set. Turning now todata set 422, in this example transitions are located at tap number 2 ofsample set 1, tap number 15 of sample set 1, tap number 14 of sample set2 and tap number 0 of sample set 4. Thus, this example includes twotransitions in sample set 1 and no transitions in sample set 3. Inprocedure 300 the method again starts in the normal state 302, and thusthe first transition in set 1 would be offset to the range T:2T−1. Thesecond transition in set 1 would then be offset to the range 0:T−1.Because two transitions were located in set 1, procedure 300 moves tothe double edge state 304. Thus, the first transition in set 2 is offsetto the range 0:T−1. Then because there is no transition in set three,the procedure 300 moves back to the normal state 302. Then the firsttransition in set 4 is offset to the range T:2T−1. Thus, in thisspecific example the first transition would be offset by 16, the secondtransition would be offset by 0, the third transition would be offset by0 and the fourth transition would be offset by 16.

The tables 430 and 431 illustrate the result of this accumulation.Specifically, the table 430 includes a column 424 of the four measuredtap numbers, while column 426 shows the four corresponding offsets. Thefirst value in column 424, 0010, is a tap number of 2. Prefixing bits 01to this value results in an offset tap number of 010010, effectivelyoffsetting the tap number by 16. The second value in column 424, 1111,is a tap number of 15. Prefixing bits 00 to this value results in anoffset tap number of 001111. Table 431 shows the running values in theaccumulator when the appropriate offset is prefixed to each to the tapnumbers and loaded into the accumulator. When the appropriate offset isprefixed to each of the other two tap numbers and loaded in theaccumulator, the result is 0111111 as illustrated in entry 428. Becauseof the way the offset values were selected, the average of all four tapnumbers can be determined using a relatively simple bit shift.Specifically, the average of all four tap numbers can be obtained bydropping the lowest two significant bits and using the next foursignificant bits 429. Thus, the average of the four tap numbers is 1111,which corresponds to a value of 15. Again, this average of the four tapnumbers corresponds to the average location of the transition in the bitstream. To locate the eye center of data in the bit stream this value isshifted one half of a bit width. Thus, by simply offsetting andaccumulating four measured tap numbers the location of the eye centercan be quickly obtained.

To further streamline this process, it is possible to directly calculatethe eye center from the accumulated values, without requiring a separateoperation to translate one-half bit width of the accumulator output.This can be accomplished by initializing the accumulator with a valuethat corresponds to the number of transitions in the data times one-halfof the bit width. When the accumulator is initialized with such a value,dropping the lowest two significant bits and using the next foursignificant bits will result in an accumulator output valuecorresponding directly to the eye center of the bits.

Furthermore, the result can be further refined to compensate for thedifference in time between samples in the sample sets. Specifically,each sample actually represents a portion of the bit time. For example,with 16 samples, each sample actually represents 1/16 of the bit time.To more accurately determine the precise eye center of the bit, it isdesirable to assume that the transition actually occurred in the middleof the sample gap before the transition was detected, rather than toassume that the transition occurred at one edge of the gap. Thisassumption can be implemented by initializing the accumulator with anadditional amount equal to the number of taps equivalent to one-half ofthe time between samples multiplied by the number of transitions. Whendivided by dropping the lowest significant bits, this results in anaccumulator output that corresponds more accurately to the eye center ofthe bit stream. Thus, by initializing the accumulator with a value tocompensate for the sample time, and to translate to the eye center, thesystem can directly and accurately determine the eye center.

Turning now to data set 442, in this example samples were located in thesample at tap number 14 of sample set 1, tap number 2 of sample set 3,tap number 15 of sample set 3 and tap number 15 of sample set 4. Thus,this example includes one transition in sample set 1, no transitions insample set 2, two transitions in sample set 3 and one transition insample set 4. In procedure 300 the method again starts in the normalstate 302, and thus the first transition in set 1 would be offset to therange T:2T−1. Because there are no transitions in sample set 2, theprocedure moves to the zero edge state 306. Thus, the first transitionin set 3 is offset to the range 2T:3T−1. Then, the second transition inset 3 is offset to the range T:2T−1. Because there were two transitionsin set 3, the procedure 300 returns to the normal state 302. Thus, thetransition in set 4 is offset to the range T:2T−1. Thus, in thisspecific example the first transition would be offset by 16, the secondtransition would be offset by 32, the third transition would be offsetby 16 and the fourth transition would be offset by 16.

The tables 450 and 451 illustrate the result of offset and accumulation.Specifically, the table 450 includes a column 454 of the four measuredtap numbers, while column 456 shows the four corresponding offsets. Thefirst value in column 454, 1110, is a tap number of 14. Prefixing bits01 to this value results in an offset tap number of 011110, effectivelyoffsetting the tap number by 16. The second value in column 454, 0010,is a tap number of 2. Prefixing bits 10 to this value results in anoffset tap number of 100010, effectively offsetting the tap number by32. The appropriate offset is likewise prefixed to each of the other twotap numbers. Additionally, table 451 shows an initial value 460 loadedinto to the accumulator to compensate for the sample time and totranslate to the eye center directly. Table 451 then shows the runningvalues in the accumulator when the appropriate offset is prefixed toeach of the tap numbers and loaded into the accumulator.

Specifically, in this example there are four transitions in the dataset, and the initial value is thus equal to 4 times one-half the bittime plus 4 times the time between samples. In this example, one-half ofa bit time is equal to 8, and 4 times this value is 32. The time betweensamples is 1 tap number, and 4 times one-half of this value is 2. Thus,the accumulator is initialized with a value of 34, or 100010, asillustrated in entry 460. When loaded in the accumulator with theaccumulated and offset tap numbers the result is 10100000 as illustratedin entry 458. Because of the way the offset values were selected andaccumulated, and because of the initial values loaded to theaccumulator, the average of the eye center can be determined directlyusing a relatively simple bit shift of this output. Specifically,dropping the lowest two significant bits and using the next foursignificant bits 461 results in the tap number that corresponds to thebit eye center. In this case, four bits are 1000, which corresponds to avalue of 8. This value corresponds directly to the eye center of thedata bits in the data stream.

It should be noted that while in the above three examplesfour-transitions are located and used to determine the bit eye center,that more or fewer transitions can be used. For example, the system canuse two transitions. When using two transitions, a bit shift of one bitin the accumulated data will provide the average location, although withlower accuracy. As another example, the system could use eighttransitions, and use a bit shift of three bits in the accumulated data.This would provide increased accuracy, but may not be feasible in caseswhere there are not eight transitions available for data training orthere is not time to wait for the analysis of eight transitions.Furthermore, the greater accuracy might not be required by the systemwhich uses the recovered data.

Turning now to FIG. 8, a schematic view of an exemplary eye centertraining system 500 is illustrated. The eye center training system 500performs both an initial training to locate the eye center and alsoincludes a retrain controller used adjust the location of the eyecenter. The system 500 is one example of the type of system that can beto initial train and retrain to recover data from a data stream.

The eye center training system 500 is particularly applicable to burstmode communication systems. In such systems multiple clients (e.g., 32clients) transmit on the same fiber or wire, each during an assignedtime slice. Each transmission in a time slice is generally referred toas a burst. Because these clients can be very remote they will typicallynot be in phase with the receiver or with each other, and the phasedifferences between clients and receivers will be unknown. Because therelative phases of clients and receivers are unknown, the trainingsystem needs to be able to accurately determine the location of bits foreach burst of data. Otherwise it cannot be assured that the incomingdata is sampled at the correct locations, and thus the reliability ofthe sampling and accuracy of the recovered data cannot be assured.

In a typical burst mode communication system, each burst of data isformatted into a packet that includes a set number of preamble data bitsfollowed by a larger number of actual data bits. The preamble data bitsprovide the transitions that are to be used by the system to recognizethe incoming data and locate the eye center. These preamble bitstypically have a known pattern, such as alternating transitions betweenevery bit, with that known pattern used for training. In some cases, thefirst preamble bits are used by the receiver's amplifier and thus only aportion of the preamble bits are available for eye center training.Because of the limited number of transitions available for training, itis desirable for the system to provide the ability to locate the eyecenter using only a few bits of data or within a few transitions. Thisallows the system to locate the eye center using the available preamblebits, before actual data bits that need to be sampled for data recoveryand passed to the receiver's system begin to arrive.

Then, after the initial training is performed the system 500 providesthe ability to adjust the bit eye center during the remainder of theburst. This allows the system 500 to compensate for drift in the datastream from the original located position of the eye center. Thus, thesystem 500 can compensate for small differences in clock speed betweenthe transmitter and the receiver that could otherwise cause potentialerrors in the data sampling and recovery.

In this illustrated embodiment, the training system 500 receives datafrom an optical data stream, such as from a burst mode Passive OpticNetwork (PON). Of course, the system could be adapted to receive datafrom other types of networks, including non-optical networks. Thetraining system 500 will be described as working for data signals of 155MHz, 622 MHz, 1.2 GHz and 2.4 GHz. Of course, these are just examples ofthe types of data stream frequencies to which the system could beapplied, and some implementations could thus include more, less ordifferent frequency capability.

The eye center training system 500 includes a sampler 504, a clock 506,a sample FIFO 508, a train controller 510, a retrain controller 512, astall FIFO 514, an accumulator 516, and a data FIFO 518. An optical datastream is received by an optical diode 501 and trans-impedance amplifier(TIA) 502. The optical diode receives the optical signal and converts itinto an appropriate electrical signal. The trans-impedance amplifier 502receives the electrical data signal and amplifies it, passing theamplified data stream to the sampler 504. It should be noted that oneissue in determining the eye center of bits in the data stream is thepresence of odd/even noise in the data stream. In this example onepotential source of odd/even noise is limitations in the performance ofthe trans-impedance amplifier 504. Because the system 500 averages thelocations of the consecutive detected transitions and uses the averagesto determine the eye center of the bits, the system is able to negatethe effects of odd/even noise. Thus, the system 500 is able toaccurately determine the eye center of bits in the data stream even inthe presence of odd/even noise.

The clock 506 provides multiple phases of a clock signal to the sampler504. The phases of the clock are used to determine when the sampler 504samples the data stream. In one specific embodiment, the clock comprisesa phase locked loop that generates 16 phases of a 622 MHz clock. Theclock phases are passed to the sampler, which uses those phases tosample the data stream.

In one embodiment, the sampler uses the 16 phases to take 32 samples ofthe data stream during each clock cycle of 622 MHz clock. It should benoted that in this embodiment the sampler would take 128 samples per bitfor a 155 MHz data stream, 32 samples per bit for a 622 MHz data stream,16 samples per bit for a 1.2 GHz data stream and 8 samples per bit for a2.4 GHz data stream.

The samples are passed from the sampler 504 to the sample FIFO 508. Thesample FIFO 508 stores the samples long enough for the system todetermine which sample is the correct sample to use for data recovery.In one embodiment the samples are passed to the sample FIFO 508 ingroups of 32, and the sample FIFO 508 stores 24 groups of 32 samples.

The sample FIFO 508 outputs to the train controller 510, the retraincontroller 512, and the stall FIFO 514. The train controller 510receives the samples from the sample FIFO 508 when a new burst startsand continues receiving samples until training completes. The traincontroller 510 uses these samples to determine the location of the eyecenter in those bits. The retrain controller 510 and stall FIFO 514receive samples from the output of the sample FIFO 508 after initialtraining completes. Thus, the retrain controller 510 and the stall FIFO514 do not receive the samples until the system is able to determine theeye center for those data bits.

In some embodiments, only a selected portion of the samples or taps isused by the train controller 510. For example, a portion of the samplesare selected to provide a selected number of samples per bit. In onespecific embodiment the samples are selected such that 16 samples areprovided to the train controller 510 for each bit for 155 MHz datastreams, 622 MHz data streams, and 1.2 GHz data streams, while 8 samplesare provided for each bit for 2.4 GHz data streams. This helps simplifythe operation of the train controller 510 by allowing the sameprocedures to be applied for different rate data streams.

As described above, in this embodiment the sampler takes 128 samples perbit for a 155 MHz data stream. Because only 16 of these samples are usedby the train controller 510, the system is only using one out of everyeight samples at this speed. Likewise, because the sampler takes 32samples per bit for a 622 MHz data stream, the system uses only one outof two samples at this data speed. Because the sampler takes 16 samplesper bit for a 1.2 GHz data stream the system uses all of the samples atthe this speed. Finally, because the sampler takes 8 samples per bit fora 2.4 GHz data stream the system uses all of the samples at this dataspeed.

It should be noted that in such a system, at the slower speeds eachsample represents a greater amount of time. As a result, the requiredaccuracy of the analysis is much lower. For example, at 155 MHz, eachbit is 6.4 ns wide, while at 2.4 GHz, each bit is 400 ps wide. Using 16samples for 155 MHz means that the samples are spaced every 400 ps. Outof 6.4 ns, an error of a few hundred ps is typically acceptable so usingonly 16 samples is sufficient. However, applying the sample rate to 2.4GHz would result in only one sample per bit, which is clearlyinsufficient. For higher speed data streams more closely spaced samplesare desirable in order to maintain the same margin of error. In oneembodiment, 16 samples are used for each bit at the speeds of 155 MHz,622 MHz, and 1.2 GHz, while only 8 samples are used for 2.4 GHz due tothe decision to limit the required frequency at which the sampler has tooperate. While this reduces the accuracy of the results at 2.4 GHzrelative to the results at the other frequencies, it can be anacceptable risk for some applications.

Each of these groups of samples is passed to the train controller 510 asa set of samples. As was described above, the train controller 510 thenlocates the transitions by identifying the samples between which thetransitions occur, and offsets the tap numbers corresponding to thetransitions based on state criteria and the number of transitions in asample set that corresponds to a bit time. The criteria and offsets usedby the train controller 510 are selected to facilitate accumulation ofthe offset tap numbers in the accumulator 516 in such a way that the eyecenter of the bits is easily located. Specifically, the criteria andoffsets are selected such that eye center of the bits can be located byusing higher order bits of the accumulated tap numbers in theaccumulator 516.

In the illustrated embodiment, the accumulator 516 is loaded withinitial values 520 to facilitate direct eye center determination and tocompensate for the difference in time between the selected samples. Asdescribed above, direct eye center determination can be facilitated byinitializing the accumulator 516 with a value that corresponds to thenumber of transitions used times the number of taps corresponding toone-half of the bit width. To compensate for the difference in timebetween selected samples, the accumulator 516 is further initializedwith an additional amount equal to number of transitions used timesone-half of the number of taps between samples.

In the specific embodiment being discussed there are four transitions inthe preamble that are used for training and the initial value is thusequal to 4 times the number of taps corresponding to one-half the bittime plus 4 times the number of taps between samples. When the samplesare added in the accumulator 516 with the appropriate offsets andinitial value the average of the eye center is determined directly usinga bit shift of this output. Specifically, dropping the proper number ofleast significant bits and using the required number of significant bitsresults in the sample tap number that corresponds to the bit eye centeras determined from four transitions. These selected bits correspond tothe eye center of the data stream and are passed to the data FIFO 518,where it they are used to identify which samples in each group ofsamples are to be used for data recovery from the data stream.

Using the specific examples introduced above, at 155 MHz, there are 128samples per bit time. Four times half of this is 256. There are 8 tapsbetween samples that are used, so four times half of this is 16. Thisyields a total initial value of 272, or, in binary, 100010000. The eyecenter can then found by dropping the two least significant bits andusing the next 7 significant bits, as the seven bits are used to selectthe one tap of 128 that corresponds to the bit eye center.

At 622 MHz, there are 32 samples per bit time. Four times half of thisis 64. There are 2 taps between samples that are used, so four timeshalf of this is 4. This yields a total initial value of 68. However,further optimization can be achieved by using the fact that 622 MHz isexactly four times 155 MHz and 68 times 4 is 272. Thus, by shifting themeaning of things by two bits, the circuitry can be shared with the 155MHz circuitry. Hence, the same initial value of 272 can be used for 622MHz when compensated for by using different significant bits todetermine the bit eye center. Specifically, using the same initial valueof 272, the bit eye center can be taken from two bits further to theleft than the answer for the 155 MHz case. The eye center at this datarate is thus found by dropping the four least significant bits and usingthe next five significant bits, where five bits are required to selectone tap of 32.

At 1.2 GHz, there are 16 samples per bit. Four times half of this is 32.There is 1 tap between samples that are used, so four times half of thisis 2. This yields a total initial value 34. Again, realizing that 1.2GHz is 8 times 155 MHz, and 34 times 8 equals 272, shifting by threewill again allow reuse of the circuitry and the same initial value of272. The eye center at this data rate is thus found by dropping the fiveleast significant bits and using the next four significant bits, wherefour bits are required to select one tap of 16 and the same relative tapis used for both bits received per sample clock.

At 2.4 GHz, there are 8 samples per bit. Four times half of this is 16.There is 1 tap between samples that are used, so four times half of thisis 2. This yields a total of 18. Again, realizing that 2.4 GHz is 16times 155 MHz, and that 18 times 8=288, shifting by four allows reuse ofthe circuitry and the same initial value of 272 if a small risk of errordue to the difference between 288 and 272 is acceptable. If the smallrisk is not acceptable, then an initial value of 288 could be used. Ineither case, the eye center at 2.4 GHz is thus found by dropping the sixleast significant bits and using the next three significant bits, withthe three bits used to select one tap of 8, and the same relative tap isused for all four bits received per sample clock

The data FIFO 518 receives the samples though the stall FIFO 514 andselects the samples that will be used for data recovery based on theoutput of the accumulator 516. Thus, the data FIFO 518 selects thesamples that correspond to the eye center of the data, as indicated bythe selected higher order bits of the accumulator 516 output. Therecovered bits are accumulated into data words of specified widths andpassed out of the data FIFO 518 to the data output 522. The size ofthese data words would typically depend on the rate of the incoming datastream and the required output data rate. For example, if the desiredoutput clock frequency is 155 MHz, for 155 MHz data, the data FIFO 518will output one bit per 155 MHz period. For 622 MHz data, the data FIFO518 will output four bits per 155 MHz period. For 1.2 GHz, the data FIFO518 will output eight bits per 155 MHz period. For 2.4 GHz, the dataFIFO 518 will output 16 bits per 155 MHz period.

The data FIFO 518 also serves to buffer the data stream to provideoutput data at a constant rate. The data FIFO 518 would typically beconfigured to accumulate some amount of data before it begins to outputdata. This allows the data FIFO 518 to supply output data at a desiredrate even if the transmitter is sending data slower than normal.Additionally, the data FIFO 518 would provide the ability to accumulatedata if the transmitter is sending data faster than normal. Thus, thedata FIFO 518 compensates for overrun and underrun by the transmitter,buffering the output to provide the receiver with the expectedconsistent stream of data.

As described above, the sample FIFO 508 also outputs to the retraincontroller 512 and the stall FIFO 514. The retrain controller 512operates after the eye center has first been located by the traincontroller 510. Thus, the retrain controller 512 operates to correct fordrift that may occur in the data stream burst as later data bits arereceived. In general, the retrain controller compares the locations ofthe transitions with the expected transition location and adjusts thesample point or tap number used for data recovery accordingly. The stallFIFO 514 buffers the samples and passes the samples to the data FIFO518. The purpose of the stall FIFO 514 is to stall the sample data suchthat it arrives at the data FIFO 518 at the same time as a decision isreached by the retrain controller 512 for that data. Thus, the retraincontroller 512 and stall FIFO 514 provide the ability to accuratelylocate the eye center of data bits throughout the burst of data.

Turning now to FIG. 9, an exemplary embodiment of a retrain controller600 is illustrated schematically. The retrain controller 600 isexemplary of the type of retrain controller that can be used inretrainer 100 illustrated in FIG. 1, and training system 500 illustratedin FIG. 8. The retrain controller 600 includes a transition detector602, a shifter 604, an expected value comparator 606, difference valueregisters 608, value selection logic 610, paired value adder(s) 612,Markov chain control logic 614, and a Markov chain 616.

In general, the retrain controller 600 receives samples of the datastream from the sample FIFO 620, determines if the current bit eyecenter used to sample the data stream is still valid, and dynamicallyadjusts the sample point to more accurately sample the data stream.

Samples of the data stream are received at the sample FIFO 620. Thesample FIFO 620 receives the samples by set, and stores several sets ofsamples at any given time. The sample FIFO 620 passes the samples by setto the transition detector 602. In one exemplary embodiment the sampleFIFO 620 receives sets of 32 samples. The rate that sets of samples aredelivered would typically depend upon the rate of the sampler clock usedto generate the samples. As one example, the sample sets could arrive ata rate of 622 MHz as determined by the sampler clock. It should be notedthat in this example the number of possible transitions in the sampleswould depend upon the data rate of the incoming data stream. It shouldalso be noted that the number of samples per data stream bit time wouldalso thus likely vary. Using the above example embodiment, the sampleFIFO 620 would receive 128 samples per bit time for 155 MHz data, 32samples per bit time for 622 MHz data, 16 samples per bit time for 1.2GHz data, and 8 samples per bit time for 2.4 GHz data.

The transition detector 602 compares the samples in each set to locateany transitions in the set. This can be accomplished by XOR'ing eachpair of adjacent samples in the set. The result of this operation is atransition bit that identifies if a transition occurred at that tapvalue. Specifically, this results in a “one” transition bit at every bitlocation where the sample value differed from the previous value, and a“zero” transition bit every where else. Of course, this is just onemethod of determining and indicating the presence of a transition in thesamples. For example, the system could instead place a “one” transitionbit where the sample value differs from the next value, and a “zero”every where else. Again, the number of transitions located in a set ofthese transition bits would typically depend on the rate of the incomingdata.

A set of transitions is passed to the shifter 604. In order to reducedevice complexity it is desirable in some applications to equalize thenumber of transition bits used at some data speeds. For example, in oneembodiment, only 16 transition bits are used per data bit for the threeslowest data rates (155 MHz, 622 MHz and 1.2 GHz). This simplifiesdevice complexity, while providing acceptable accuracy. For the fastestrate of 2.4 GHz, this embodiment uses 8 samples per data, an acceptablecompromise for accuracy and complexity. The number of transition bitscan be reduced by “OR'ing” sets of adjacent bits. When this operation isperformed, only the set of adjacent bits that includes the “one”transition bit will result in a value of one, and the rest will remainat zero. For example, the 128 transition bits in one bit time at 155 MHzcan be reduced by OR'ing adjacent sets of eight bits together, resultingin a reduced set of 16 transition bits. The 32 bits in one bit time at622 MHz can be reduced to 16 transition bits by OR'ing pairs of adjacenttransition bits. At 1.2 GHz and 2.4 GHz all of the original transitionbits would be used.

Thus, a set of transition bits is passed to the shifter 604. The shifter604 loads the transition sets into a register. The shifter 604 shiftsthe transition sets such that the expected location of the transitionsin the set are at specific, known locations in the register. Thisfacilitates quick determination of movement in the bit eye center. Itshould be noted that shifting greatly simplifies retraining by takingadvantage of the realization that only one transition is going to befound between any two samples. This means that retraining can beaccomplished without attempting to account for multiple transitionsbetween samples, greatly simplifying the logic needed to retrain.

Turning briefly to FIGS. 10-13, schematic views of sets of transitionbits and a shift register are illustrated. Specifically, FIG. 10illustrates a first set (set 1) of 32 transition bits 702 and a 64-bitregister 708. In this example, the transition bits 702 include onetransition bit 704 corresponding to a transition that was located in theset of samples. Also illustrated in transition bits 702 is the bitlocation of expected eye center 706. Register 708 is illustrated showingthe expected transition location 710 and the expected eye centerlocation 712.

As described above, shifter 604 loads the transition bits into aregister shifted such that the expected location of the transitions inthe transition set are at specific known locations in the register.Turning now to FIG. 11, the register 708 is shown loaded with the firstset of 32 transition bits. The transition bits 702 are loaded into theregister 708 such that the bit location of the expected eye center 706is loaded to the expected eye center location 712. The expected eyecenter 706 will vary with the actual location of the data bit relativeto the sampler clock, but the expected eye center location 712 willremain fixed. As can be seen in FIG. 11, the located transition bit 704is one bit to the right of the expected location 710. This indicatesdrift in the eye center of the data stream.

As more sets of transition bits come in they are loaded into theregister, and the previous bits are shifted left. Turning now to FIG.12, a second set (set 2) of transition bits 722 is illustrated alongwith the 64-bit register 708 that already includes the previously loadedtransition bits 702. In this example, the transition bits 722 includeone transition bit 724 corresponding to a transition that was located inthis set of samples. Also illustrated in transition bits 722 is the bitlocation of expected eye center 706.

Again, the shifter 604 loads the transition bits into a register shiftedsuch that the expected location of the transitions in the transition setare at specific known locations in the register. Furthermore, previouslyloaded transition bits are shifted to the left to accommodate the newbits, as illustrated by arrows 750. Turning now to FIG. 13, the register708 is shown loaded with the second set of 32 transition bits 722. Ascan be seen in FIG. 13, the located transition bit 724 is two bits tothe left of the expected location 710.

The first set of transition bits 702 has been shifted left, with only aportion of the first set 702 remaining in the register 708. It should benoted that in this example, the transition bit now at location 726corresponds to the bit in the first set 702 that was at the expectedlocation of the eye center. Typically, the shifted transition bits wouldbe passed to the expected value comparator 606 in sets that extend fromexpected eye center to expected eye center, or stated another way, fromsample point to sample point minus one. This facilitates easydetermination of movement in the eye center of the data stream.

Specifically, shifting and loading the transition bits such that theexpected value of the transition is in a known place facilitates the useof relatively quick logic to determine the direction and magnitude ofhow far the transition is from where it was expected. For example, ifafter loading into the register the actual location of the transition istwo bits from the expected location, then the magnitude of movement canbe easily determined using simple logic to be 2, with the signdetermined by the direction of movement. Conversely, without this shifta more computationally complex and time consuming subtraction would berequired to determine the movement.

It should be noted that the expected value of the transition isdetermined by the current bit eye center that is being used to recoverdata. Specifically, the expected value of the transition is half a bittime from the sample point. Thus, this value would initially bedetermined by the training system and could also have been previouslyupdated by the retrain controller 600.

Returning to FIG. 9, the expected value comparator 606 compares theactual location of the transitions with the expected value of thetransitions, and outputs a difference value representing the differencebetween the expected location and the actual location. In oneembodiment, the difference value is a positive or negative number,depending on the direction of difference. Thus, if the actual transitionlocation is one bit to the left of the expected location, the differencevalue would comprise negative 1. If the actual transition location istwo bits to the right, the difference value would comprise positive 2.If the actual transition location is at the expected location, then thedifference value would comprise zero. Thus, in the example of FIGS.10-13, the difference value for transition bit 704 would comprisepositive 1, and the difference value for transition bit 724 wouldcomprise negative 2.

It should be again noted that while the examples illustrated in FIGS.10-13 show only one located transition in each transition bit set, thatthis would not always be the case. Again, in the example discussed abovefor 1.2 GHz, data as many as two transitions will occur every sampleset, and for 2.4 GHz data as many as four transitions will occur everysample set. In those cases the expected locations of the transitionswould be separated by 16 or 8 bits, and the expected value comparator606 could compare each located transition with its expected value in theregister simultaneously, and would create a plurality of differencevalues during each clock cycle.

Returning to FIG. 9, the difference values are passed to the differencevalue registers 608. The difference value registers 608 store differencevalues as they are generated by the expected value comparator 606, andmake the difference values available to the value selection logic 610.

The rate at which difference values become available in the differencevalue registers 608 again depends upon the incoming data rate and thenumber of transitions in the sampled portion of the data stream. Thus,at 155 MHz it will take at least 8 cycles of a 622 MHz sample clock togenerate a pair of difference values. In fact, it may take significantlymore cycles due to the fact that data stream could include manyconsecutive data bits without transitions between them. When adifference value is loaded in the difference value registers 608 and noother difference value is available, that “odd-numbered” differencevalue is stored in the register until the next transition and itsdifference value arrives. In general, you need one more difference valueregister than the number of transitions which can occur in one clockcycle.

At 2.4 GHz it is possible for two pairs of transitions to be availableeach 622 MHz clock cycle. Again, in some cases due to drift orconsecutive bits without transitions, an odd number of difference valueswill arrive during a clock cycle. In those cases the last remainingdifference value is again kept in the difference value registers 608until the next transition arrives and a difference value is available topair with.

Thus, the value selection logic 610 selects pairs of difference valuesfrom the difference value registers 608 as they become available.Specifically, the value selection logic 610 selects consecutive pairs ofdifference values. Thus, the value selection logic 610 selects adifference value for a 0 to 1 transition and a 1 to 0 transition,regardless of how many bits of data have been between the transitions.

The pair value adders 612 sums the selected pair or pairs of differencevalues together. When added together, the pairs of difference valuesproduce an adjustment value that corresponds to how far the current eyecenter used to sample data is from the actual eye center. For example,if the first transition is one bit to the left, its difference valuewould be negative 1, and if the second transition is two bits to theleft its difference value would be negative 2, and the sum of the pair,the adjustment value, would be negative 3. As a second example, if thefirst transition is three bits to the right, its difference value wouldbe positive 3, and if the second transition is two bits to the left, itsdifference value would be negative 2, and the sum of the pair would bepositive 1. These adjustment values are then used to selectively shift aMarkov chain to adjust the sample point, as will be discussed in greaterdetail below.

It should be noted that if both difference values in a pair are 0, or ifthey have the same magnitude with an opposite sign, then the sum of thepair, and hence the adjustment value, will be 0. A zero adjustment valuecorresponds to the “center,” meaning that the current sample point iscorrect based on those transitions.

It should also be noted that the use of a sum of a pair of differencevalues compensates for the presence of odd/even noise in the datastream. Specifically, because a pair of difference values is used, aconstant amount is effectively subtracted from half the numbers and thesame amount is added to the remaining half. Thus, the resulting averageis not affected by the odd/even noise and the retrainer is able toaccurately determine the eye center of bits in the data stream.

The paired value adders 612 preferably include as many adders as neededto handle the largest number of pairs which can be generated in oneclock cycle. In the example discussed above, an adder would be used toadd difference values at 155 MHz, 622 MHz, and 1.2 GHZ. At 2.4 GHz it ispossible to get two pairs of difference values per sampler clock. Thus,for 2.4 GHz an adder is used for one pair, and a second adder would beused to add the second pair of difference values that could be generatedin one clock cycle at this speed. Using the second adder allows thesecond pair to be added in parallel with the first pair. Otherwise therewould be the potential to lose data at this speed.

The adjustment value(s) are passed to the Markov chain control logic614. The Markov chain control logic 614 uses the adjustment values toshift right or shift left the Markov chain 616, which is in turn used toadjust the sampling tap used by the data FIFO 622 to recover data. TheMarkov chain control logic 614 preferably looks at the magnitude andsign of the adjustment value to determine how to shift the Markov chain616. As one example, if the adjustment value is positive, the Markovchain control logic 614 shifts the Markov chain 616 to the right.Conversely, if the adjustment value is negative, the Markov chaincontrol logic 614 shifts the Markov chain 616 to the left.

The Markov chain control logic 614 can look at the magnitude of theadjustment value to determine how much to shift the Markov chain 616. Inone example, the Markov chain control logic 614 is configured to selectone magnitude if the adjustment value is less than ¼ of a bit width, andto double the shift if the adjustment value corresponds to greater than¼ of a bit width and less than ½ of a bit width. The bit width again isdetermined by the data rate. So at 155 MHz, ¼ of a bit width wouldcorrespond to a difference value of 4 (when a reduced set of 16transition bits is used). Thus, if the adjustment value is greater than4 (or less than −4) the Markov chain control logic 614 would doubleshift the Markov chain 616. Adjustment values that correspond to morethan ½ a bit are actually on the other side of the sample point. Thus,these transitions may actually correspond to an adjacent bit transition.

The Markov chain 616 is a shift register which effectively acts as a lowpass filter. Specifically, when training completes the Markov chain 616is initialized with a “1” in the center position and a “0” in all otherpositions. It is then shifted left or right by the Markov chain controllogic 614 as adjustment values are received by the logic 614. Thus, inthe 155 MHz embodiment discussed above, if the adjustment value is 0,the Markov chain 616 is not shifted. If the adjustment value is positive1, 2 or 3 the Markov chain 616 is shifted one to the right. If theadjustment value is negative 1, 2, or 3, the Markov chain 616 is shiftedone to the left. If the adjustment value is greater than 3, the Markovchain 616 is shifted two to the right. Finally, if the adjustment valueis less than negative 3, the Markov chain 616 is shifted two to theleft.

The accumulated effects of multiple shifts can cause the Markov chain616 to reach the right or left end and “roll-off”. When the Markov chainrolls off, it moves the sample point used to recover the data in thedata FIFO 622, thus retraining to a new determined bit eye center. Asone example, the Markov chain 616 comprises a 17 element chain. In thisembodiment the Markov chain 616 would be initialized with only the9^(th) bit set, and it would take accumulated adjustment values of ±9 tocause the Markov chain 616 to roll-off.

When a roll-off occurs, the Markov chain 616 is reinitialized to thecenter position. Additionally, since a roll-off changes the bit eyecenter used by the system, the shifter 604 is updated with the newexpected transition values that result from the updated eye centervalue. It should be noted that the retraining makes the remaining datain the retrain controller 600 in error. Specifically, because theremaining data in the difference value registers 608, value selectionlogic 610, pair value adders 612 and Markov chain control logic 614 wasgenerated using the previous value for the eye center, it is now off bywhatever amount the eye center was adjusted. To compensate for aroll-off in the Markov chain 616 and facilitate the use of thisremaining data, the Markov chain control logic 614 preferably offsetsthe adjustment values to compensate for the change until new, postchange data arrives. Thus, if the Markov chain 616 rolls-off to theleft, and thus increments the eye center, the remaining data would alsobe shifted left to compensate.

For example, if the Markov chain 616 rolled-off to the left the “center”adjustment value would now be a negative 2. It is negative 2 becauseeach expected transition point has moved left one spot. Hence each “old”value would be off by negative 1, both taken together would be off bynegative 2. Negative 2 would then be the new base value that should beused by the Markov chain control logic 614 to determine if, and by howmuch, the Markov chain 616 should be shifted. For example, an adjustmentvalue of 0 would then be interpreted as if it where a positive 2, andthe Markov chain would be shifted one to the right. The new center ofthe adjustment value would continue to be used until new data based onthe adjusted sample point arrives at the Markov chain control logic 614.When new, post sample point adjustment data, begins to arrive the centerof the adjustment value is moved back to zero.

The retrain controller thus 600 compares the locations of thetransitions with the expected transition location and adjusts the samplepoint or tap number used for data recovery in the data FIFO 622. Thus,the retrain controller 600 provides the ability to accurately locate theeye center of data bits throughout the burst of data.

The embodiments and examples set forth herein were presented in order tobest explain the present invention and its particular application and tothereby enable those skilled in the art to make and use the invention.However, those skilled in the art will recognize that the foregoingdescription and examples have been presented for the purposes ofillustration and example only. The description as set forth is notintended to be exhaustive or to limit the invention to the precise formdisclosed. Many modifications and variations are possible in light ofthe above teaching without departing from the spirit of the forthcomingclaims.

1. An eye center retrainer, the eye center retrainer comprising: asampler, the sampler receiving a data stream and taking a plurality ofsamples of the data stream; a retrain controller, the retrain controllerreceiving the plurality of samples, the retrain controller locatingtransitions in the plurality of samples, comparing the locatedtransitions to an expected location, and generating difference valuesbased on the comparing, the retrain controller combining pairs ofdifference values corresponding to consecutive transitions to determinehow to adjust a sample point used for data recovery from the datastream.
 2. The eye center retrainer of claim 1 wherein the retraincontroller further includes control logic, the control logic adjustingthe sample point based on a sign and magnitude of the combined pair ofdifference values.
 3. The eye center retrainer of claim 2 wherein theretrain controller includes a Markov chain, wherein the control logicshifts the Markov chain based on the sign and magnitude of the combinedpair of difference values.
 4. The eye center retrainer of claim 3wherein the control logic shifts the Markov chain a first amount whenthe magnitude of the combined pair of difference values corresponds toless than ¼ of a data bit width, and wherein the control logic shifts asecond amount double the first amount when the magnitude of the combinedpair of difference values corresponds to more than ¼ of the data bitwidth and less than ½ of the data bit width.
 5. The eye center retrainerof claim 2 wherein the control logic adjusts a center point used toevaluate the combined pair of difference values when the sample pointhas been adjusted to compensate for effects of the sample pointadjustment.
 6. The eye center retrainer of claim 5 wherein the controllogic returns the center point to a previous value when a new combinedpair of difference values based on the adjusted sample point arrives. 7.The eye center retrainer of claim 1 wherein the retrain controllerfurther includes a shifter, the shifter shifting the plurality ofsamples such that an expected location of the transitions loads to aknown specified location.
 8. The eye center retrainer of claim 1 whereinthe retrain controller further comprises difference value registers, thedifference value registers storing difference values to facilitatepairing of difference values corresponding to consecutive transitions.9. The eye center retrainer of claim 1 wherein the combining pairs ofdifference values comprises summing the pairs of difference values. 10.A method for retraining an eye center, the method comprising: receivinga data stream and taking a plurality of samples of the data stream;locating transitions in the plurality of samples; comparing the locatedtransitions to an expected location of the transitions; generatingdifference values based on the comparing; and combining pairs ofdifference values corresponding to consecutive transitions to determinehow to adjust a sample point used for data recovery from the datastream.
 11. The method of claim 10 wherein combining pairs of differencevalues corresponding to consecutive transitions to determine how toadjust a sample point used for data recovery from the data streamcomprises adjusting the sample point based on a sign and magnitude ofthe combined pair of difference values.
 12. The method of claim 11wherein adjusting the sample point based on a sign and magnitude of thecombined pair of difference values comprises shifting a Markov chainbased on the sign and magnitude of the combined pair of differencevalues.
 13. The method of claim 12 wherein shifting a Markov chain basedon the sign and magnitude of the combined pair of difference valuescomprises shifting the Markov chain a first amount when the magnitude ofthe combined pair of difference values corresponds to less than ¼ of adata bit width, and shifting a second amount double the first amountwhen the magnitude of the combined pair of difference values correspondsto more than ¼ of the data bit width and less than ½ of the data bitwidth.
 14. The method of claim 10 further comprising adjusting a centerpoint used to evaluate the combined pair of difference values when thesample point has been adjusted to compensate for effects of the samplepoint adjustment.
 15. The method of claim 14 further comprisingreturning the center point to a previous value when a new combined pairof difference values based on the adjusted sample point arrives.
 16. Themethod of claim 10 further comprising shifting the plurality of samplessuch that an expected location of the transitions loads to a knownspecified location.
 17. The method of claim 10 further comprisingstoring difference values to facilitate pairing of difference valuescorresponding to consecutive transitions.
 18. An eye center retrainer,the eye center retrainer comprising: a sampler, the sampler receiving adata stream and taking a plurality of samples of the data stream; aretrain controller, the retrain controller receiving the plurality ofsamples, the retrain controller comprising: a shifter, the shiftershifting the plurality of samples into a register such that an expectedlocation of the transitions loads to a known specified location; anexpected value comparator, the expected value comparing locations oftransitions in the register to the expected location to generatedifference values; a difference value register, the difference valueregister storing the difference values; value selection logic, the valueselection logic combining pairs of difference values corresponding toconsecutive transitions in the data stream; and chain control logic, thechain control logic summing pairs of difference values to generateadjustment values, chain control logic shifting a chain in response tothe adjustment values to adjust a sample point used for data recoveryfrom the data stream based on a sign and magnitude of the summed pair ofdifference values.
 19. The eye center retrainer of claim 18 wherein thechain control logic shifts the chain a first amount when the magnitudeof the combined pair of difference values corresponds to less than ¼ ofa data bit width, and wherein the control logic shifts the chain asecond amount double the first amount when the magnitude of the combinedpair of difference values corresponds to more than ¼ of the data bitwidth and less than ½ of the data bit width.
 20. The eye centerretrainer of claim 18 wherein the chain control logic adjusts a centerpoint used to evaluate the adjustment values when the sample point hasbeen adjusted to compensate for effects of the sample point adjustmentand wherein the control logic control logic returns the center point toa previous value when an adjustment value based on the adjusted samplepoint arrives.